Early data in the literature suggests that the total numbers of Epstein- Barr viral genomes increase in the peripheral blood of organ transplant patients prior to the diagnosis of post-transplant lymphoproliferative disorders (PTLD). This study will attempt to corroborate thus data and further attempt to detect emerging EBV-infected clonal populations in the peripheral blood of these patients. It is preusmed that detection of increasing Epstein-Barr viral load and/or emerging clonal populations will define a group of patients who would benefit from decreased immunosuppressive therapy.